The present invention relates to an outer race for a constant velocity joint, having improved anti-flaking properties and shaft strength, and a process for producing the same.
Regarding outer races for constant velocity joints, one of drive components of automobiles, in order to cope with an increase in power of automobile engines or atmospheric control in recent years, there is an ever-increasing trend toward an increase in strength of outer races. Strength properties required of outer races for constant velocity joints are anti-flaking properties and shaft strength. For the outer races, track grooves in a cup, which come into contact with balls, should have anti-flaking properties. The strength properties required of the shaft are torsional strength and torsional fatigue strength.
Japanese Patent Laid-Open Publication No. 280262/1997 discloses a high strength outer race for a constant velocity joint, and a process for producing the same. According to the technique disclosed in this laid-open publication, a steel comprising carbon: 0.45 to 0.53%, silicon: 0.05 to 0.25%, manganese: 0.7 to 1.0%, aluminum: 0.01 to 0.05%, molybdenum: 0.2 to 0.4%, and nitrogen: 0.003 to 0.012%, and, in addition, chromium: limited to not more than 0.05%, phosphorus: limited to not more than 0.015%, sulfur: limited to not more than 0.01%, and oxygen: limited to not more than 0.002% is first forged in the temperature range of 850 to 950xc2x0 C. to form a section having a regulated fine grain structure. The section is then cold forged, machined, and form rolled into a predetermined shape including an involute serration and the like, followed by induction hardening and tempering of the involute serration. Thus, a high strength outer race for a constant velocity joint is obtained wherein the involute serration in its end has a former austenite grain size of not less than 8 in terms of the grain size number specified in JIS (Japanese Industrial Standards), the surface hardness HV is not less than 720, and the ratio of the effective case depth to the radius of the shaft is 0.35 to 0.60.
The above publication refers to the improvement in torsional strength and torsional fatigue strength of the shaft. The anti-flaking properties (described as the rolling life in the above publication), however, are equal to those attained in the prior art. Further, forming is mainly carried out by hot forging. This is likely to cause significant distortion in the track grooves, to cause partial contact of balls, and to cause a deterioration in anti-flaking strength.
The technique disclosed in the above publication cannot provide excellent anti-flaking properties as the outer race for a constant velocity joint without difficulty. Accordingly, it is an object of the present invention to solve the above problems of the prior art and to provide an outer race for a constant velocity joint, which is excellent in both anti-flaking properties and shaft strength, and a process for producing the same.
The above object can be attained by the following invention. Specifically, according to one aspect of the present invention, there is provided an outer race for a constant velocity joint, including a cup and an involute serration and having improved anti-flaking properties and shaft strength, said outer race comprising as a steel material by weight carbon: 0.45 to 0.59%, silicon: 0.15 to 0.4%, manganese: 0.15 to 0.45%, sulfur: 0.005 to 0.15%, molybdenum: 0.1 to 0.35%, boron: 0.0005 to 0.005%, aluminum: 0.015 to 0.05%, and titanium: 0.015 to 0.03% and, in addition, nitrogen: limited to not more than 0.005% (inclusive of 0%), chromium: limited to not more than 0.1% (inclusive of 0%), phosphorus: limited to not more than 0.02% (inclusive of 0%), and oxygen: limited to not more than 0.002% (inclusive of 0%), with the balance consisting of iron and unavoidable impurities, the proportion of the effective case depth in track grooves of the cup being 0.25 to 0.45 in terms of the ratio of the effective case depth t to the wall thickness of the cup w, t/w, the proportion of the effective case depth of the involute serration in its end being 0.20 to 0.50 in terms of the ratio of the effective case depth t to the radius r, t/r. Preferably, the involute serration in its end has a former austenite grain size of not less than 8 in terms of the grain size number specified in JIS (Japanese Industrial Standards).
According to another aspect of the present invention, there is provided a process for producing an outer race for a constant velocity joint, including a cup and an involute serration and having improved anti-flaking properties and shaft strength, said process comprising the steps of: subjecting a steel comprising the above constituents to annealing, cold forging, machining, and form rolling to form an outer race having a predetermined shape; and then induction hardening the outer race in its shaft including the inner surface of the cup and the involute serration, followed by tempering, thereby producing such an outer race for a constant velocity joint that the proportion of the effective case depth in track grooves of the cup is 0.25 to 0.45 in terms of the ratio of the effective case depth t to the wall thickness of the cup w, t/w, and the proportion of the effective case depth of the involute serration in its end is 0.20 to 0.50 in terms of the ratio of the effective case depth t to the radius r, t/r. Preferably, the former austenite grain size of the involute serration in its end is brought to not less than 8 in terms of the grain size number specified in JIS.
Use of the outer race for a constant velocity joint and the process for producing the same according to the present invention can provide constant velocity joint products possessing improved anti-flaking properties and shaft strength.